Atmospheric pressure ion sources coupled to mass spectrometers often produce random noise spikes or significant ion loss which can severely limit the signal-to-noise ratio in the mass spectra. Ion transfer tubes or capillaries are well known in the field of mass spectrometry for the transport of ions between an ionization chamber maintained at or near atmospheric pressure and a second chamber maintained at reduced pressure. Generally described, an ion transfer channel typically takes the form of an elongated narrow tube (capillary) having an inlet end open to the ionization chamber and an outlet end open to the second chamber.
Several ion funneling solutions have been proposed in the art. For example, an ion funnel for operation under vacuum conditions after an ion transfer capillary was described in U.S. Pat. No. 6,107,628. Unfortunately, most ion funnel devices only operate effectively up to gas pressures of approximately 40 mbar. While some of these approaches may be partially successful for reducing ion loss and/or alleviating adverse effects arising from ion collisions with the tube wall, the focusing at atmospheric pressure with minimal noise remains a challenge.
There remains a need for atmospheric pressure ion guides capable of focusing ions for a variety of ion detection devices and from a variety of ion sources.